Treatment osi bock asphalt



Patented Apr. 3, 19285 VTILLIAM P. BENTLEY, OF DALLAS, TEXAS.

TREATMENT OF ROCK ASPHALT.

No Drawing.

vThis application is a continuation. in part of my .co-pendingapplication, Serial, No. 152,723.

The present invention has for its principal objects the improvement inmethods for the commercial utilization of naturally occurring rockasphalt, and the improvement thereby of the quality of the product forwhich the material is to be adapted. The principal use ofthe materialhere contemplated is for that of paving streets, roadways, highways,foot-paths, mastics for fioorings, roofings, and the like, but in itsbroader aspects might be utilized for any purpose for which asphalticmaterials have been used.

. Rock asphalts are found in a great variety of forms and in variouslocalities, and are both siliceous and calcareous in body impregnatedwith bituminous material ranging from very hard to very soft. By far thegreater percentage of these rock asphalts is of such composition thatthey cannot be used per se for pavements, but require pretreatment tosome extent. This treatment may comprise crushing the rock asphalt, theliquefaction of the self-contained bitumen either by means of heat or bya solvent and the subsequent or simultaneous addition of a bituminousfiux for the purpose of softening or hardening the bitumen and at thesame time increasing the bitumen content to that desired in the finalpavement.

Heretofore the methods used for regulating the crushing operation andfor computing the proper amount and kind of bituminous llux for theproduction of the greatest stability or strength in the final pavementhas been entirely haphazard and mere guess work. As an illustration ofthe present state of the art, in a bulletin issued September i, 19:26,by the A. & at. College of Texas, J. T. L. McNeW, professor of highwayei'igineering, after an exhaustive discussion of Texas rock asphalts,concludes that The proper fiuxing of the bitumen in Texas limestone rockasphalt that is lean in natural asphalt content is a problem whichcannot be solved without further investigation. It is precisely such anextensive investigation carried on by me over a Application filedFebruary 17, 1928. Serial No. 255,202.

period of years which has led to the concept which is the embodiment ofthis invention.

The specific feature of this invention is primarily that of regulatingthevoidage in the crushed rock asphalt aggregate, either by the crushingoperation itself, or by the addition of certain desirable size ofbitumenfree or bitumen-impregnated particles, or by both. Concurrentlywith this, by knowing or PIQ-Cl6t8l'lllll1ihg' voidage, the selection ofthe suitable flux for imparting the desired hardness or softness to thenaturally contained bitumen becomes a matter of scientific control, andis always productive of improved quality to the final product. Thiscombination of voidage regulation, flux consistency, and qualitycontrol, has never been known before, nor has it ever been possiblebefore to pie-determine the stability of the final composition. With agiven kind of rock asphalt which may have been in use for a long periodof years, experience may have led to very successful results forvarieties of conditions. Even current practice which has given practicalresults, can be greatly improved by application of my method andmixtures can then be controlled as to characteristicsheretofore.mentioned. But when attempting to open new deposits or towork with differentkinds of rock asphalts, nothing in past experiencewould be available, hence controlled stability would be a matter ofassumption or' mere guess work. This invention also opens up vast fieldsof rock asphalt which have been considered as inadequate and inferior onaccount of not lend ing itself to successful operation when the samekind of flux as is ordinarily used, is employed; likewise, rock asphaltscarrying bitumen of different degrees of consistency were not deemedsatisfactory for the usual given method of treatment. In order toillustrate one method of using this invention, a description of designin a mixture for paving purposes may be given; this refers to aparticular kind of rock asphalt found in Texas, but in no manner limitsthis invention to such particular kind of rock as phalt or particularkind of usage.

The rock asphalt is crushed in any ordinary type of mill or roll, andwith its self contained bitumen constituting the aggregate, its voidageis determined by compaction to refusal in any of the usual instrumentsfor such a determination. Then various proportions of previouslyseparated particles, for instance, 200 mesh sizes, or 80 mesh, or 40mesh, or 10 mesh, or such, may be added in progressive increments untilthe minimum practicable combination of voids for workability isproduced. From this voidage and the specific gravity of the rockasphalt, the amount of bitumen necessary to be added to fill said voidscan be readily computed. With this valuable information the kind ofbitumen to be added to keep the resulting consistency of the finalproduct within specified limits can be determined by simple fluxingtests. Then, by determining the stability values of ccmpres. ed mixtureseither by shear tests, or other such method, a predetermined servicerecord will be available. By knowing the traffic, climatic conditions,etc, to be encountered, any of the above factors can be regulated oradjusted tofmeet the proposed need. I have discovered that the highestpracticable shear values are obtained by so regulating the bitumen thatthe voids in the final composition, after compression, are ust filled.Which, stated in another way, means that the amount of bitumen in thefinal mixture is regulated by adding, if necessary, a flux in such anamount that, when taken in conjunction with the self-contained bitumenin the rock, fills the voids without substantial excess or deficiency.

The most common failure which might be found inherent in an asphalticpavement is that of distortion, and since the distorting forces oftrailic are mainly shear, it can be seen that resistance to shear is adirect measurement of the resistance of a pavement to distorion.

From the above it is believed that those skilled in the art will betaught how to obtain the highest practicable shear values. The engineerknows that the rock asphalt contains naturally a certain amount ofbitumen to which a flux must be added in order that the sum may fill thevoids. The

part added must be less than the sum so above. Take Example A. It isstated that:

(V) Percent voids in rock asphalt as nsed= 23.2.

(P) Percent bitumen in .rock asphalt=8 to 9.

T Percent computed bitumen content required I 10.35.

(Percentage total bitumen required to fill voids in aggregate).

The mixture actually contained 10.3%

bitumen.

lVhat amount of flux was added? Obviously, it was 10.3% minus thepercent bitumen already contained in the rock. In other words, 1.3% to2.3% would be required for rock asphalt containing 8 to 9 percentbitumen.

Not only does this above outlined procedure enhance the quality of knownrock asphalts, but it also opens up for use other vast deposits whichhave been ignored for supposed lack of sufficient bitumen, or seeminglyimproper degree of crushing resistance, or too hard or too softself-contained bitumen, or a combination of these qualities, which by mytreatment are rendered highly satisfactory.

The following tabulation will assist in an understanding of the presentinvention:

EXAMPLE A.

Regular rm! Una/dc rock asphalt containing 8 to 9% bitumen. Percentagevoids in rock asphalt as (Refers to voids and grading of crushed rockasphalt with its self-contained bitumen present.)

Computed bitumen content required 7 (Percentage total bitumen requiredto fill voids in aggregate.)

Shear strength at 140 F. in pounds per square inch 63. 3

Mixture analysis (after shearing sample). Bitumen 10.3 Per cent passing200 mesh 20. 3 80 mesh 14. 5 40 mesh; 19. (3 10 mesh 26. 8 4 mesh. 8. 52 mesh 0 Approximate penetration at 77 of hi- The mixture actuallycontained 10.3% bitumen. The flux added was 10.3% minus: the percent ofbitumen already contained in the rock. In other words 1.3% to 2.3% wouldbe required for rock asphalts containing 8 or 9% bitumen.

tumen extracted from mixture EXAMPLE B.

RegnIani-nn Unalde rock asphalt containing 8 to 9% bitumen witlt' 200mesh portion increased 10%no flnansed.

Percentage voids inrock asphalt V 18. 3

Sieve analysis of rock asphalt percent age passmg- 2 00 meshu "I 14.5mesh 11.

40 mesh 19.7 10 mesh 40.3 a mesh 11.1 2 mesh 3.3

(Refers to voids and grading of crushed rock asphalt with itsselficontamed bitumen present) Computed bitumen content required 7. 9

(Percentage total bitumen required to fill Voids in aggregate). Shearstrength at 140 F. in pounds per square inch 190. Mixture analysis(after shearing sample). Bitumen 9.0

Percent passing: 200 mesh 80 mesh 40 mesh 10 mesh 1 4 mesh 2 mesh[Oi-FLO wewwe LOOQOQDO Approximate p netration at 77 F. of

bitumen extracted from mixture 8 This example is included to show thatwhen the voids are reduced to 18.3%, rock asphalt containing 8% bitumenalready has its voids filled. It is impossible to flux this rock asphaltto raise the penetration of the bitumen to a higher and more desirableworking penetration without overfilling the voids, unless the bituminouscontent thereof is lowered by blending with leaner rocks or by addingmineral matter. If the penetration of the self-contained bitumen wasalready at the desired point, all that would be necessary touse thisrock under this method would be to reduce the voids until theselfcontained bitumen just filled same. The fiux to be added in such acase would be 0%.

EXAMPLE 0.

Regular run Utalde rock asphalt containing 1.1% total bitumen.

Percentage of voids, sieve analysis, and computed bitumen contentrequiredidenti- Cal with Example A.

The mixture analysis, however, showed 11% bitumen with a shear of 27.7and a penetration in excess of 100.

Note from Example C what happens to the shear as compared with Example Awhen the voids are overfilled by the total bitumen.

I Here the amount of flux added was too much by 0.7%. This mix was madethis way purposely for illustration.

EXAMPLE D.

Using only the same base with, only 10% bitumenhad a shear strength of56.5, a bitmncn content in me mix-titre analysis of 9.!) and apenetration of 30.

Note from this example what happens to the shear when the voids areunderfilled by total bitumen. Here the flux added was too little.

ExAMPLE E.

Special lean Uvalde rock asphalt of 5.5 to 6% bitumen with flux added inan amountsuiiicient to produce in the final mixture a total bitumencontent from .5% to 1.0% in excess of that actually required to fillvoids.

Percentage voids in rock asphalt 18. 6

Sieve analysis of rock asphalt passing:

Approximate penetration at 77 F. of hitumen extracted from mixturel00plus.

Note from Example E that, when the voids (V) are reduced to 18.6% andthe total bitumen is one-half to one percent in excess and using theflux generally used on Texas rock asphalt hot mixes, the shear is only21.7 pounds. Note how the total bitumen required as computed goes downto 8.05% with lowering of voids. The high penetration accounts for thelow shear shown by this example and discloses that when low voidagen'ii-xtures are used 011 Texas rock asphalts, the flux now in generaluse is not 01" the right type.

EXAMPLE F.

Same as Example E with .no excess oil llux, using 300 penetration fluxshowed an increase in shear strength to 111.5, and a decrease inpenetration to 26.

ExAMrLn G.

Special lean Uruldc rock asphalt of 5.5 lo (i btlameu, with 200 HtGi/tincreased and hi lumen 10 fill voids (using pcncfrulioa flur).

per square inch -r 1 .5. 0

Mixture analysis showed 7.9% bitumen having a penetration ol 2%.

Note in this example in Examples E and F) when voids are reduced, it isnecessary for correct design to obtain special lean Uvalde rock asphaltin order that the total bitumen in the mixture may not be too high. Herewith 18.0% voids the total bitumen required was computed to be T=7.7%.P. said to be to (l%with a 5.5% bituminous rock the flux required was,of course, 22%. Lool: at the strength obtained-14t5 pound shear. Actualanalysis showed 7.9% bitumen in the mix which is in close agreement withthe design.

This example shows correct design tor low voidage, for percent totalbitumen, and for selection of flux for controlling penetration. Itshould be contrasted with Example E, in

which the flux was not properly selected.

The above tabulations will clearly illustrate the broad scope of thepresent invention. For specific illustration, Example F utilizes a rockasphalt containing approximately only 6% bitumen, which as beforepointed out, had never been employed to advantage as a base for pavingmaterial, tor supposed lack of bitumen content. Rock asphalt with thislean content of bitumen, develops as indicated, excellent shearvaluewhen fluxed with the proper penetration mate 'ial.

15y first detern'iining the voidage of the crushed rock asphalt and theamount and penetration of the bitumen therein, the amount andpenetration of the flux to be added thereto may be definitelyascertained, the amount being based on that necessary to fill the voids,and the penetration being based on the final peiietration desired relative to the amount added. It is necessary to determine the amount ofbitumen contained in the crushed rock asphalt because the volume of thevoids is to be filled by the volume of the bitumen contained .inthe rockasphalt plus the volume ol the added flux, and we cannot know the amountof flux required unless we know both the voids and the amount of bitumenin the rock asphalt. The penetration may readily be held where it shouldbe for best results .in actual practice, and the amount offiux requiredis consistent with present 'fiuxing ratios.

it may be pointed out that high stability and shear can be obtained bymixing the socalled lean (containing 5 to 6% bitumen) and the regularrun rock asphalt (containing between S and 10% bitumen).

The penetration of the bitumen in the final mixture is an important itemso far as mixture stability is concerned, regardless oi the kind of fluxemployed.

A comparison between Example A and Example F will indicate that byregulation of the crushing, the percentage of voids in the rock asphalta'l'ter crushing, has been reduced :l'rom 23.2 to 18.6. This decrease inthe voidage may be also broijight about instead oit, or in addition to,controlling the crushing operation as indicated in Example G, in whichthe voids in aggregate have been reduced to 18.0% by adding a, definiteamount of desirable sized. particles, which gradmg may not be obtainedsolely by crushit may be pointed out that the percentage of bitumen inthe final mixture is materially less in the mixture utilizingarelatively lean rock asphalt as compared with the mixtures using therelatively rich rock asphaltsr The resulting economy in the use oilesser quantities of flux tobe added will be immediately apparent. Thisis obvious because in the light of the foregoing context it is seen thatwhen the voids are reduced. less bitumen is required to fill them. )Vithany grade of rock asphalt with a given percent bitumen, loweringthevoids means simply that less flux is added to the contained bitumen tofill the voids. Vith low void mixtures only lean rocks can be used,otherwise the voids are overfilled.

Since it is known that the most des' 'able stability values are inherentin such mix tures as have just sufiicient bitumen to fill the voids,(the bitumen in the mixture is, of course, the naturally containedbitumen in the rock asphalt plus the bitumen flux), and since the voidshave been reduced in the mixtures described. it is apparent that thesaving in flux is equal to the volume reduction in voids. Reducing thevoids in any rock asphalt simply means that the total bitumen requiredis less. In any'rock asphalt of a given bituminous content the fluxrequired to be added to the naturally contained bitumen is less whenvoids are reduced. Again, if the voids are just filled with the totalbitumen, it is obvious that the part added in the form of flux is, initself, insufficient to fill the voids, but in the mixtures designed asdescribed in the examples cited, the bituminous fiux added will, whenadded to the bitumen alreadycontained in the rock as phalt, just fillsaid voids without substantial excess or deficiency.

It is strikingly shown that when mixtures are properly designed on avoidage basis, a total bitumen content of .5 of 1% more than thetheoretically correct amount of material reduces the shear strengthmaterially.

' It has been found that mixtures are most stable when they contain thetheoretically correct amount of bitumen. This sharp decline in resultingstability produces, of course, an inferior paving mixture.

Mixture stability is definitely cont-rolled by virtue of the grading ofthe crushed rock asphalt. The lower the voids, the greater thestability, provided the penetration of the bitumen is constant.

It is obvious from each of the examples given that the differencebetween the computed bitumen content required to fill the voids in theaggregate and the self-contained bitumen inthe rock asphalt is theamount of flux necessary to be added. The specification states that thevoids are found in order tn compute the total bitumen required to fillthem. These examples state the amount of bitumen already in the rock andthe difference is to be made up of flux, of course.

The preparation of a mixture in accordance with the specificationsoutlined above may be carried out in any standard paving plant underapproved methods as will be obvious. The flux may be added in heatedcondition and the crushed rock asphalt may or may not be heated prior tothe introduction of the flux, or, as is obvious, the mixture may beprepared cold by utilizing a volatile solvent for liquetying the addedbituminous flux and the bitumen in the crushed rock asphalt.

I claim as my invention:

1. A method which comprises first determining the voidage in rockasphalt reduced to usable sizes, then adding bituminous flux in anamount insufiieient to fill said voids but in an amount which, whenadded to the naturally contained bitumen, is sufficient, with saidnaturally contained bitumen, to fill said voids without a substantialexcess or substantial deficiency.

2. A method which comprises determiningthe voidage in rock asphalt, thenreducing said voidage by grading, it necessary, to a minimum practicabletor usability, then adding bituminous flux in an amount insulticient tofill said voids but in an amount which, when added to the naturallycontained bitumen, is sufiicient, with said naturally contained bitumen,to substantially fill said voids without a substantial excess orsubstantial deficiency.

3. A method which comprises first determining the voidage in rockasphalt reduced to usable sizes, then adding bituminous flux in anamount insufiicient to fill said voids but in an amount which, whenadded to the naturally contained bitumen, is sufiicient, with saidnaturally contained bitumen, to fill said voids without a substantialexcess, or substantial deficiency, and cont-rolling, b selection, theconsistency of said added flux to produce a mixture of a predeterminedstability in which the total bitumen content has a predeterminedpenetration.

4-. A method which comprises first determining the voidage in rockasphalt reduced to usable sizes, then adding bituminous flux in anamount insufiicient to fill said voids but in an amount which, whenadded to the naturally contained bitumen, is suilicient, with saidnaturally contained bitumen, to fill said voids without a substantialexcess or substantial deficiency, liquefying the bitumen in said mixtureand compacting the mixture in place to produce a bituminous structurepossessing high stability.

5. A method of preparing a bituminous composition which comprisesreducing rock asphalt to usable sizes and determining the voidagethereof, then reducing said voidage by grading, it necessary, to aminimum prac ticable for usability by adding thereto bitumen-free orbitumen-impregnated mineral matter of desired sizes, and then adding abituminous flux thereto in an amount insufficient to fill said voids butsufiicient, when added to the bitumen naturally contained in the rock,to fill said voids without substantial excess or substantial deficiency.

In testimony whereof I aiiix my signature.

l/VILLIAM P. BENTLEY.

